The effects of fire on soil nitrogen associated with patches of the actinorhizal scrub Ceanothus cordulatus

Abstract:

Nitrogen is a limiting resource in many temperate forests and nitrogen-fixing plants are usually limited to the early
stages of post-disturbance succession. In fire-dependent Sierra Nevada forests, however, Ceanothus cordulatus is
relatively abundant even in old-growth forest conditions which are at least partly maintained by fire. We conducted
a field experiment to determine if soil beneath Ceanothus patches represent ‘resource islands’ of available N which
persist after fire. Nine plots containing discrete patches of Ceanothus, Arctostaphylos patula (manzanita; chosen
as a non N-fixing reference species), and bare forest floor were subjected to either a low-intensity (n = 3) or high intensity (n = 3) bum treatment, or remained unburned as controls (n = 3). Soil temperatures during the bum were
monitored by a network of thermocouples placed at the surface of the mineral soil and at ca. 10 cm depths. Soil
samples were collected from the organic horizon, 0-10 cm and 15-25 cm depths within each patch type immediately
before burning and 2 days, and 6, and 11 months after. Soil moisture, total C and N, and ammonium and nitrate
concentrations were determined in the laboratory. Before the burn, Ceanothus patches were significantly enriched
in total and inorganic N in the organic horizon relative to the other patch types. A sharp increase in inorganic N was
observed in all patch types and depths immediately following burning, but by 6 months after the burn, Ceanothus
patches were significantly enriched relative to the surrounding patch types and remained so at months. Resprouting
Ceanothus patches will continue to be an important source of a limiting nutrient in this fire-prone ecosystem.